Motor for an electric power steering assembly

ABSTRACT

A motor for an electric power steering assembly comprises a yoke, a multi-polar magnetic field portion composed of at least four poles secured to the inner wall of the yoke, a shaft disposed within the yoke so as to be able to rotate freely, an armature secured to the shaft having a winding constructed by winding wiring into an even number of slots formed on the outer circumferential surface of a core so as to extend in the axial direction thereof, a commutator comprising a plurality of segments secured to an end portion of the shaft; and a plurality of brushes contacting the surface of the commutator.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a motor for an electric powersteering assembly for assisting the steering force of an automotivesteering wheel.

[0003] 2. Description of the Related Art

[0004]FIG. 15 is a cross-section of a conventional motor for an electricpower steering assembly (hereinafter “electric motor”) 100. The electricmotor 100 comprises: a cylindrical yoke 1; two permanent field magnets 2arranged circumferentially and secured so as to face each other insidethe yoke 1; a shaft 4 disposed inside the yoke 1 by means of bearings 3so as to be able to rotate freely; an armature 5 secured to the shaft 4;a commutator 6 comprising a plurality of copper segments 16 secured toan end portion of the shaft 4; and brushes 8 placed in contact with thesurface of the commutator 6 by the elastic force of springs 7.

[0005] The armature 5 comprises: a core 9 having a plurality of slots 11extending in the axial direction; and a winding 10 constructed bywinding wiring into the slots 11 by a lap winding method.

[0006] In the above 2-pole lap-wound electric motor 100, an electriccurrent is supplied to the winding 10 from outside by means of thebrushes 8 contacting the segments 16, whereby the armature 5 rotatestogether with the shaft 4 due to electromagnetic action.

[0007] Since the above electric motor 100 is mainly used in relativelylight-weight low-capacity automobiles, the assisting torque from theelectric motor 100 is small and consequently the operating noise of theelectric motor 100 is extremely small so small that it is practicallyunnoticeable inside the automobile.

[0008] However, now that fuel-conservation and weight reduction arerequired even in heavy-weight middle- and high-capacity automobiles dueto public demand for fuel efficiency, reduced exhaust emissions, etc.,direct-current motor power steering assemblies are starting to replacehydraulic power steering assemblies. Electric motors providing largetorque are required in such cases, but since 2-pole lap-wound designsresult in large-bodied motors, it is necessary to increase the number ofpoles to four or so to reduce size and produce high torque.

[0009]FIGS. 16 and 17 show comparisons between a 2-pole 14-slotdirect-current motor (hereinafter “2-pole motor”) and a 4-pole 21-slotdirect-current motor (hereinafter “4-pole motor”) given as an example ofa multi-polar machine. These figures show the differences in magneticattraction acting on the armatures in 2-pole and 4-pole motors when thearmatures are off center and were obtained by magnetic field analysis bythe present inventors. In FIG. 16, “” represents the center of thestator, that is, the original center of rotation, and “x ” representsthe center of rotation when off center. In FIG. 17, “

” represents the force of eccentricity direction, “

” represents the force of right angle thereof. As can be seen from thefigure, vibrations and noise are generated more easily in a 4-pole motorthan in a 2-pole motor.

[0010] That is, when the forces acting on the armatures were examinedwith each being placed off center by the same amount (0.1 mm) from theoriginal central position in every angle of eccentricity from 0 degreesto 360 degrees, the maximum magnetic attraction acting in the directionof eccentricity in the 4-pole motor was approximately 2.7 N, or sixtimes the maximum magnetic attraction acting in the direction ofeccentricity in the 2-pole motor which was approximately 0.45 N. In the2-pole motor, the direction of magnetic attraction due to eccentricitycan be clearly seen, and when the force acting is compared to the angleof eccentricity it is found that when the eccentricity is between thepoles (an angle of eccentricity of 90 degrees or 270 degrees)approximately twice as much magnetic attraction (0.45/0.21) acts as whenthe eccentricity is directed towards the center of a pole (an angle ofeccentricity of 0 degrees or 180 degrees). In the 4-pole motor, on theother hand, no clear direction can be seen. That is, the force in thedirection of eccentricity is approximately 2.7 N for every angle ofeccentricity from 0 degrees to 360 degrees, which means that there is adirection of stability with respect to eccentricity in a 2-pole motor,but no such direction exists in a 4-pole motor, and this difference canbe considered to be related to the differences in vibration and noise.

[0011] Thus, it is necessary to increase the number of poles to four orso in order to reduce size and produce high torque, but problems ofvibration and noise remain.

[0012] Now, apart from lap winding, wave winding may also be consideredas a winding method for armatures when the number of poles is increasedin order to reduce size and increase torque. With a lap winding, thenumber of brushes provided is generally the same as the number of poles,but with a wave winding two brushes are generally provided.

[0013]FIGS. 18 and 19 are sets of diagrams and graphs showing themagnetic attraction acting on a 4-pole 21-slot armature given as anmulti-polar example, FIG. 18 showing a case with a lap winding and fourbrushes and FIG. 19 showing a case with a wave winding and two brushes.In FIGS. 18 and 19, “

” represents 100% current flows perpendicular to the paper in an upwarddirection, “

” represents 100% current flows perpendicular to the paper in andownward direction, “

” represents 50% current flows perpendicular to the paper in an downwarddirection, “

” represents 50% current flows perpendicular to the paper in an upwarddirection, and “{circle over (O)}” represents current does not flow.

[0014] Comparing the two figures, we see that whereas in the case ofwave winding the magnetic attraction acting on the armature as thearmature turns by one slot of the core is always directed in a givenradially-outward direction as indicated by the arrow A, in the case of alap-wound 21-slot armature, the magnetic attraction movescircumferentially as indicated by the arrow B, and one problem with alap-wound 21-slot armature is that rotational vibrations arise easily,making the generation of operating noise that much more likely.

[0015] In the case of a multi-polar odd numbered-slot lap winding,another problem is that differences arise in the electromotive forcesinduced among the circuits of the winding of the armature due to theinfluences of imbalances in the electromagnetic circuit of the yoke,eccentricities in the armature, nonuniform electric currents flowingthrough the brushes, engineering errors, etc., giving rise tocirculating currents within the armature flowing through the brushes,and as a result the commutating action of the brushes deteriorates,leading to increases in temperature, shortened working life, increasesin torque ripples in the brushes and the commutator which accompany anincrease in commutation sparks generated by the brushes, as well as thecombined effects thereof, thereby increasing operating noise.

[0016] At the same time, in the case of a multi-pole odd numbered-slotwave winding, there are problems such as torque ripples increasing inmagnitude and workability deteriorating due to increased thickness ofthe winding in order to reduce the number of parallel circuits, etc.

SUMMARY OF THE INVENTION

[0017] The present invention aims to solve the above problems and anobject of the present invention is to provide a motor for an electricpower steering assembly enabling reduced operating noise.

[0018] In order to achieve the above object, according to one aspect ofthe present invention, there is provided a motor for an electric powersteering assembly comprising: a yoke; a multi-polar magnetic fieldportion composed of at least four poles secured to the inner wall of theyoke; a shaft disposed within the yoke so as to be able to rotatefreely; an armature secured to the shaft having a winding constructed bylap winding wiring into an even number of slots formed on the outercircumferential surface of a core so as to extend in the axial directionthereof; a commutator comprising a plurality of segments secured to anend portion of the shaft; and a plurality of brushes contacting thesurface of the commutator.

[0019] According to one form of the present invention, there is provideda motor for an electric power steering assembly wherein the number ofslots is even and is not a multiple of the number of poles.

[0020] According to another aspect of the present invention, there isprovided a motor for an electric power steering assembly comprising: ayoke; a multi-polar magnetic field portion composed of at least fourpoles secured to the inner wall of the yoke; a shaft disposed within theyoke so as to be able to rotate freely; an armature secured to the shafthaving a winding constructed by lap winding wiring into a number ofslots being a multiple of the number of pairs of poles, the slots beingformed on the outer circumferential surface of a core so as to extend inthe axial direction thereof; a commutator comprising a plurality ofsegments secured to an end portion of the shaft; and a plurality ofbrushes contacting the surface of the commutator.

[0021] According to one form of the present invention, there is provideda motor for an electric power steering assembly wherein the number ofslots is a multiple of the number of pairs of poles and is not amultiple of the number of poles.

[0022] According to another form of the present invention, there isprovided a motor for an electric power steering assembly comprisingequalizing members for preventing circulating currents from flowingthrough the brushes due to differences in induced electromotive forcesarising between circuits within the circuits of the armature.

[0023] According to still another form of the present invention, thereis provided a motor for an electric power steering assembly whereinNs/(n×2)≦K≦Ns, where K is the number of equalizing members, Ns is thenumber of slots in the core, and n is the maximum number of segmentscovered by the brushes.

[0024] According to one form of the present invention, there is provideda motor for an electric power steering assembly wherein the currentpassing through the winding is controlled by pulse width modulation(PWM) driving.

[0025] According to another form of the present invention, there isprovided a motor for an electric power steering assembly wherein thewiring is enamel-coated round wire.

[0026] According to still another form of the present invention, thereis provided a motor for an electric power steering assembly wherein themagnetic field portion comprises a plurality of permanent magnetsdisposed so as to be spaced around the inner wall of the yoke.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a partial cross-section of a motor for an electric powersteering assembly according to Embodiment 1 of the present invention;

[0028]FIG. 2 is an enlargement of part of FIG. 1;

[0029]FIG. 3(a) is a developed front elevation of the equalizer mainbody in FIG. 1;

[0030]FIG. 3(b) is a side elevation of FIG. 3(a);

[0031]FIG. 4(a) is a front elevation of the base of the equalizer mainbody in FIG. 1;

[0032]FIG. 4(b) is a side elevation of FIG. 4(a);

[0033]FIG. 5 is a front elevation of a terminal of the equalizer mainbody in FIG. 1;

[0034]FIG. 6 is a front elevation of an insulating plate of theequalizer main body in FIG. 1;

[0035]FIG. 7 is a set of diagrams and graphs explaining magneticattraction acting on an armature having four poles, a lap winding, fourbrushes, and twenty-two slots;

[0036]FIG. 8 is a front elevation showing another example of a terminal;

[0037]FIG. 9 is a cross-section showing another example of an armature;

[0038]FIG. 10 is an enlargement of part of FIG. 9;

[0039]FIG. 11 is a graph showing the relationship between the number ofterminals and auditory evaluation;

[0040]FIG. 12 is a graph showing the relationship between motor outputclass and motor operating noise for various types of motor;

[0041]FIG. 13 is a perspective view showing a motor for an electricpower steering assembly mounted on a pinion;

[0042]FIG. 14 is a graph showing the relationships between control gain,fluctuation in torque, and magnetic attraction in a radial direction;

[0043]FIG. 15 is a cross-section of a conventional motor for an electricpower steering assembly;

[0044]FIG. 16 is a set of diagrams explaining magnetic attraction in a2-pole motor and in a 4-pole motor;

[0045]FIG. 17 is a set of graphs explaining magnetic attraction in a2-pole motor and in a 4-pole motor;

[0046]FIG. 18 is a set of diagrams and graphs explaining magneticattraction and torque ripples in a 4-pole lap-wound 21-slot 4-brushmotor for an electric power steering assembly;

[0047]FIG. 19 is a set of diagrams and graphs explaining magneticattraction and torque ripples in a 4-pole wave-wound 21-slot 2-brushmotor for an electric power steering assembly;

[0048]FIG. 20 is a block diagram for a control unit;

[0049]FIG. 21 is a set of diagrams and graphs explaining magneticattraction and torque ripples in a 4-pole lap-wound 24-slot 4-brushmotor for an electric power steering assembly;

[0050]FIG. 22 is a set of diagrams and graphs explaining magneticattraction and torque ripples in a 4-pole lap-wound 20-slot 4-brushmotor for an electric power steering assembly;

[0051]FIG. 23 is a set of diagrams and graphs explaining magneticattraction and torque ripples in a 4-pole lap-wound 26-slot 4-brushmotor for an electric power steering assembly;

[0052]FIG. 24 is a set of diagrams and graphs explaining magneticattraction and torque ripples in a 4-pole lap-wound 28-slot 4-brushmotor for an electric power steering assembly;

[0053]FIG. 25 is a table showing the relationship between torque ripplesand magnetic attraction in 4-pole lap-wound 20-, 21-, 22-, 24-, 26-, and28-slot 4-brush motors for electric power steering assemblies;

[0054]FIG. 26 is a set of diagrams and graphs explaining magneticattraction and torque ripples in a 6-pole lap-wound 25-plot 6-brushmotor for an electric power steering assembly;

[0055]FIG. 27 is a set of diagrams and graphs explaining magneticattraction and torque ripples in a 6-pole lap-wound 24-slot 6-brushmotor for an electric power steering assembly;

[0056]FIG. 28 is a set of diagrams and graphs explaining magneticattraction and torque ripples in a 6-pole lap-wound 22-slot 6-brushmotor for an electric power steering assembly;

[0057]FIG. 29 is a set of diagrams and graphs explaining magneticattraction and torque ripples in a 6-pole lap-wound 26-slot 6-brushmotor for an electric power steering assembly;

[0058]FIG. 30 is a set of diagrams and graphs explaining magneticattraction and torque ripples in a 6-pole lap-wound 21-slot 6-brushmotor for an electric power steering assembly;

[0059]FIG. 31 is a set of diagrams and graphs explaining magneticattraction and torque ripples in a 6-pole lap-wound 27-slot 6-brushmotor for an electric power steering assembly; and

[0060]FIG. 32 is a table showing the relationship between torque ripplesand magnetic attraction in 6-pole lap-wound 21-, 22-, 24-, 25-, 26-, and27-slot 6-brush motors for electric power steering assemblies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Embodiment 1

[0061] An example of a motor for an electric power steering assembly(hereinafter “electric motor”) according to the present invention willnow be explained, and parts the same as or corresponding to those of theconventional example will be given the same numbering.

[0062]FIG. 1 is a cross-section of the internal construction of anelectric motor according to Embodiment 1 of the present invention, andFIG. 2 is an enlargement of part of FIG. 1. The electric motorcomprises: a cylindrical yoke 1; four permanent magnets 2 composed offerrite spaced circumferentially and secured inside the yoke 1; a shaft4 disposed inside the yoke 1 by means of bearings 3 so as to be able torotate freely; an armature 20 secured to the shaft 4; a commutator 6comprising a plurality of segments 16 secured to an end portion of theshaft 4; four brushes 8 spaced equidistantly and placed in contact withthe surface of the commutator 6 by the elastic force of springs 7; andan equalizer main body 22 secured to the shaft 4 between the armature 20and the commutator 6. Moreover, the yoke 1, permanent magnets 2,bearings 3, springs 7, and brush 8 are not shown in these figures.

[0063] The armature 20 comprises: a core 9 having twenty-four slots 11extending in the axial direction; and a winding 21 constructed bywinding wiring 19 into the slots 11 by a lap winding method.

[0064]FIG. 3(a) is a developed front elevation of the equalizer mainbody 22, and FIG. 3(b) is a side elevation of the equalizer main body 22in FIG. 3(a). The equalizer main body 22 comprises: twelve terminals 24composed of copper plate, etc., which are equalizing members; and twelveinsulating plates 25, alternately stacked in layers on a base 23.

[0065] FIGS. 4(a) and 4(b) are a front elevation and a side elevation,respectively, of the base 23. Twelve pins 26 are arranged so as to standequidistantly around the circumference of a toric base main body 27.

[0066]FIG. 5 is a front elevation of a terminal 24 being an equalizingmember. Apertures 29 are formed at 24 points spaced equidistantly aroundthe circumference of the annular terminal main body 28 of each of theterminals 24 being equalizing members. Furthermore, equalizer leadportions 30 a, 30 b extending radially outwards in opposite directionsare disposed on the terminal main body 28.

[0067]FIG. 6 is a front elevation of an insulating plate 25. Apertures32 are formed at 24 points spaced equidistantly around the circumferenceof the annular insulating plate main body 31 of each of the insulatingplates 25.

[0068] In the above electric motor, the equalizer main body 22 isassembled by alternately stacking the twelve terminals 24 and the twelveinsulating plates 25 on the base 23. During this process, eachsuccessive terminal 24 is rotated by 15 degrees and the terminals 24 aresecured to the base 23 by passing the pins 26 of the base 23 through theapertures 29 in the terminals 24. Furthermore, the insulating plates 25are secured to the base 23 by passing the pins 26 of the base 23 throughthe apertures 32 in the insulating plates 25. Then, the equalizer mainbody 22 is integrated by crimping the ends of the pins 26.

[0069] Next, the equalizer main body 22 and the commutator 6 are fittedonto the shaft 4 in that order. Protrusions 14 extending in the axialdirection are formed on the shaft 4 in order to position the equalizermain body 22 and the commutator 6 relative to the direction of rotation,and the base 23 and the commutator main body 15, which are both composedof phenol resin, are secured to the protrusions 14 by elasticdeformation.

[0070] Next, the armature 20 is formed by bending the equalizer leadportions 30 a, 30 b to align with hooks 34, and forming the winding 21by winding the wiring 19 onto the core 9 by a lap winding method, thenthe equalizer lead portions 30 a, 30 b and the hooks 34 are electricallyconnected at twenty-four points by simultaneous fusion or the like.

[0071] An electric motor of the above construction has four magneticpoles, twenty-four slots 11, a lap winding, and a 4-brush system. FIG.21 is a set of diagrams and graphs of magnetic attraction and torqueripples acting on the armature 20 in the above motor which the presentinventors obtained by magnetic field analysis. Whereas in the case ofthe 4-pole lap-wound 4-brush, 21-slot armature of FIG. 18 describedabove, the magnetic attraction acting on the armature movescircumferentially and rotational vibrations arise easily, making thegeneration of operating noise that much more likely, it is clear that inthe case of a 24-slot lap-wound armature, the total magnetic attractionacting on the armature is zero and that operating noise therefore doesnot arise due to rotational vibrations.

[0072]FIG. 7 is a set of diagrams and graphs of magnetic attraction andtorque ripples acting on an armature having four poles, a lap winding,and an even numbered twenty-two slots which the present inventorsobtained by magnetic field analysis.

[0073] As can be seen from the figure, in the case of a 22-slotlap-wound armature, the total magnetic attraction acting on the armatureis also zero and operating noise therefore does not arise due torotational vibrations.

[0074] Furthermore, whereas in the case of the wave-wound armature ofFIG. 19 described above, the torque ripples (p-p) represented by theratio of vertical variance in the torque wave to the total torque are1.37 percent, in the case of the 22-slot lap-wound armature the torqueripples (p-p) are smaller than the wave-wound case at 0.876 percent. Forthat reason, in an electric motor 18 driven by pulse width modulation(PWM) by means of a motor drive signal from a control unit 13 as shownin FIG. 20, the torque ripples are reduced, improving the feel of thesteering wheel 12 to the driver compared with a wave-wound electricmotor.

[0075] Moreover, annular terminal main bodies 28 are used in anequalizer main body 22 of the above construction, but arc-shapedterminal main bodies 50 may be used in terminals 52, as shown in FIG. 8,in order to conserve the amount of copper material used.

[0076] Furthermore, as shown in FIGS. 9 and 10, six terminals 24 and sixinsulating plates 25 of an equalizer main body 60 may be alternatelystacked on the base 23 and a terminal 24 electrically connected to everysecond hook 34, or a terminal may be electrically connected to everythird hook 34.

[0077] In order to prevent circulating currents from flowing through thebrushes due to differences in the induced electromotive force arisingbetween the circuits, the greater the number of terminals beingequalizing members the greater the effect, but as explained above, thenumber may be reduced to allow for easier production and lower costs forthe equalizer main body.

[0078] It was found that operating noise was smallest when the number ofterminals satisfied the equation Ns/(n×2)≦K≦Ns, where K is the number ofterminals, Ns is the number of slots in the core, and n is the maximumnumber of segments covered by the brushes. FIG. 11 is an evaluation forthe case where Ns=22 and n=3, and the above formula satisfies theevaluation criteria, where six or more out of ten is passable.

[0079] Furthermore, in an electric motor of the above construction,machine winding of the wiring 19 of the winding 21 using enamel-coatedround wire is possible in order to reduce production costs and enablemass production, but even a wiring machine cannot wind in perfect rowsand there is a risk that irregularities in the resistance and inductancebetween circuits of the winding will increase. However, becausecirculating currents are prevented from flowing through the brushes dueto differences in the induced electromotive force arising between thecircuits by the provision of the equalizer main body 22, problemsarising from irregularities in the resistance and inductance betweencircuits of the winding do not occur.

[0080] Furthermore, in an electric motor of the above construction,permanent field magnets 2 composed of ferrite are used in order toreduce the torque ripples most associated with steering. When the fieldis generated by electromagnets, the magnetic flux density is generallyhigher than that of permanent magnets, intensifying the changes in fluxdensity in the gap as the slots and the teeth of the core alternatelyface the poles due to changes in position in the direction of rotationof the armature, thereby increasing torque ripples. Whereas the averageflux density in the gap in the case of permanent ferrite field magnetsis normally approximately 0.3 to 0.4 Tesla, it is approximately doublein the case of electromagnets at 0.7 to 0.8 Tesla, and in the case ofelectromagnets, torque ripples increase, fluctuations in magneticattraction also increase at the teeth of the core, and electromagneticnoise also increases. Furthermore, when permanent ferrite field magnetsare used, it becomes possible to reduce the size of the motor, simplifythe assembly operation, and reducing costs.

[0081] Thus, it is effective to use permanent ferrite field magnets inan electric motor, but in that case, since the magnetic flux density ofthe field is low, it is necessary to increase the number of winds of thewiring in the armature to ensure torque quality. For that reason, thefield magnets are greatly affected by the reaction from the armature,and the magnetic center of the flux distribution of the magnetic fieldpoles is shifted greatly in the opposite direction to the rotationaldirection of the armature. In an ordinary motor, this shift in magneticcenter is compensated for by offsetting the brushes from the geometriccenter of the magnetic poles in the opposite direction to the rotationaldirection of the armature to obtain a good flux distribution. However,because this electric motor rotates in both directions, it is notpossible to compensate for shifts in magnetic center by offsetting thebrushes in the opposite direction to the rotational direction of thearmature in order to obtain a good flux distribution.

[0082] Consequently, in this electric motor, good flux distribution isensured by improving the balance of the induced voltage in each of thecircuits of the winding by providing an equalizer main body 22 on thearmature 20 in order to compensate for poor flux distribution, and thespecial effects described below are obtained.

[0083] (1) Because the operating noise of this electric motor is reducedas shown in FIG. 12, the driver does not notice any unpleasant operatingnoise while steering, even if this electric motor is mounted on thesteering column. Moreover, since this electric motor can be mounted onthe column within the automobile cabin, it is placed in a moreadvantageous environment with respect to heat and water than aconventional electric motor 100 which is mounted, for example, on a rack40 in the engine compartment as shown in FIG. 13, enabling this electricmotor to be manufactured more cheaply.

[0084] (2) Because this electric motor adopts a lap-wound 4-brushmethod, torque ripples can be reduced, and even if this motor is drivenby pulse width modulation (PWM) by means of a motor drive signal from acontrol unit 13, vibrations transmitted to the steering wheel 12 duringactivation of this electric motor are practically nonexistent,preventing deterioration of the feel of the steering wheel to thedriver.

[0085] Furthermore, because torque ripples are reduced in this electricmotor, the degree of freedom in designing the PWM driving method of thecontrol unit 13 is increased, allowing improvements in responsivenessand microcurrent control to be introduced, further improving the feel ofthe steering wheel.

[0086] Furthermore, holding noise (the noise generated by vibrationscaused by an electric motor caused by changes in torque due to changesin the current flowing through the armature resulting from minutechanges in the contact between the brushes 8 and the segments 16 whenthe steering wheel 12 is held in a given position; or the vibratingnoise generated in the period of minute displacement due to backlashfrom the system when an electric motor is not active) can be reduced. Ina conventional wave-wound 2-brush method, torque ripples are large andholding noise is easily generated, but when attempts are made tosuppress the generation of this holding noise by means of the controlunit 13 by increasing control gain, torque fluctuations indicating thedegree of holding noise are reduced as shown in FIG. 14, while operatingnoise (magnetic attraction in the radial direction) is increased, and itis not possible to suppress both holding noise and operating noisesimultaneously. On the other hand, in this electric motor employing alap-wound 4-brush method, it is possible to suppress both holding noiseand operating noise simultaneously.

[0087] (3) Because this electric motor adopts a lap-wound 4-brushmethod, the current density in the brushes 8 can be reduced, enablingthe allowable current-bearing time of this electric motor to belengthened. During reverse parking, U-turns, etc., the steering wheel 12is frequently turned to its maximum angle and used in a so-called“stationary steering” or “end locked” state, but at that time thearmature of an electric motor hardly rotates at all while torque isstill being generated, and the electric motor is used in a constrainedstate. This electric motor allows the current density in the brushes 8to be reduced at that time, when temperature increases are harshest,enabling the allowable period of use in a “stationary steering” or “endlocked” state to be lengthened, thereby increasing the utility of theelectric motor.

[0088] Furthermore, the working life of the brushes 8 is lengthenedthereby, improving the reliability and durability of the electric motor.

[0089] (4) Because this electric motor adopts a lap-wound 4-brushmethod, the cross-sectional area of the wiring in the winding 21 can beapproximately half that of a wave winding under identical conditions,facilitating shaping of the wire and improving winding, and because thediameter of the wire is small, there are fewer gaps between portions ofthe wiring within the slots 11 of the core 9, improving thewire-to-space ratio and enabling the size of the electric motor to bereduced. Consequently, the moment of inertia and torque loss of thearmature 20 which are important factors in steering can be reduced.

[0090] (5) By improving the balance of the induced voltage between eachof the circuits of the winding, an overall reduction in torque ripplescan be achieved, reducing the torque ripples transmitted to the steeringwheel, and enabling an overall improvement in the feel of the steeringwheel to the driver.

[0091] (6) Because this electric motor provides a good commutatingaction, in addition to enabling effects such as the lengthening of theworking life of the brushes 8, the suppression of temperature increasesin the brushes 8, and the reduction of commutator noise (spark noise) inthe brushes 8, it is advantageous with respect to radio noise, etc.,because the generation of sparks is reduced. In particular, when mountedon the steering column where use in close proximity to radio powercircuitry, etc., cannot be avoided, the effects on radio noise, etc.,are small.

[0092] Furthermore, because the generation of sparks is reduced, theload of the springs 7 pressing the brushes 8 against the commutator 6can be reduced, enabling the reduction of torque loss due to brushpressure, and also enabling the reduction of frictional heat due to thepressure of the brushes 8. Consequently, even though this electric motoradopts a lap-wound 4-brush method, torque loss can be maintained at thesame level as that of a wave-wound 2-brush method.

[0093] Moreover, the above embodiment was explained for 4-pole 24- and22-slot lap-wound motors for electric power steering assemblies, but thenumber of slots is not limited to these numbers, and provided that thenumber of slots is an even number which does not give rise to magneticattraction in the radial direction relative to the armature, the noisereduction effect will be realized.

[0094] Additionally, provided that the number of slots is not a multipleof the number of poles, torque ripples can also be reduced.

[0095] FIGS. 22 to 24 show the magnetic attraction and torque ripplesacting on an armature in the cases of 4-pole 20-slot, 4-pole 26-slot,and 4-pole 28-slot lap windings, and it is clear that magneticattraction does not act in the radial direction in any of these cases.FIG. 25 summarizes these results, and it can be seen that magneticattraction does not occur in the radial direction when the number ofslots chosen is an even number or a multiple of the number of pairs ofpoles, and that torque ripples can be reduced if the number of slots isnot a multiple of the number of poles.

[0096] Furthermore, the number of poles is not limited to four, and maybe any number from four upwards, such as six, eight, etc. FIGS. 26 to 31show examples of 6-pole 25-plot, 6-pole 24-slot, 6-pole 22-slot, 6-pole26-slot, 6-pole 21-slot, and 6-pole 27-slot lap windings.

[0097] From FIG. 26, it can be seen that magnetic attraction acts in theradial direction because the number of slots is neither an even numbernor a multiple of the number of pairs of poles. In FIG. 27, the torqueripples are large because the number of slots is a multiple of thenumber of poles. In FIGS. 28 and 29, the number of slots is an evennumber but not a multiple of the number of poles, and in FIGS. 30 and31, the number of slots is a multiple of the number of pairs of polesbut not a multiple of the number of poles, and so magnetic attractiondoes not act in the radial direction in any of these cases, and torqueripples are minimized. FIG. 32 summarizes these results, and as with the4-pole cases, it can be seen that magnetic attraction does not occur inthe radial direction if the number of slots is an even number or amultiple of the number of pairs of poles, and that torque ripples can bereduced if the number of slots is not a multiple of the number of poles.The same applies to cases with eight poles or more. When the number ofslots is a multiple of the number of pairs of poles, the equalizingmembers described above can be provided, enabling circulating current tobe prevented and the commutating action to be improved.

[0098] As explained above, according to one aspect of the presentinvention, there is provided a motor for an electric power steeringassembly comprising: a yoke; a multi-polar magnetic field portioncomposed of at least four poles secured to the inner wall of the yoke; ashaft disposed within the yoke so as to be able to rotate freely; anarmature secured to the shaft having a winding constructed by lapwinding wiring into an even number of slots formed on the outercircumferential surface of a core so as to extend in the axial directionthereof; a commutator comprising a plurality of segments secured to anend portion of the shaft; and a plurality of brushes contacting thesurface of the commutator, whereby the total magnetic attraction actingon the armature is zero and rotational vibrations which cause operatingnoise do not arise, thereby enabling operating noise to be reduced.

[0099] According to one form of the present invention, there is provideda motor for an electric power steering assembly wherein the number ofslots is even and is not a multiple of the number of poles, enablingoperating noise to be reduced, as well as reducing torque ripples andimproving the feel of the steering wheel to the driver.

[0100] According to another aspect of the present invention, there isprovided a motor for an electric power steering assembly comprising: ayoke; a multi-polar magnetic field portion composed of at least fourpoles secured to the inner wall of the yoke; a shaft disposed within theyoke so as to be able to rotate freely; an armature secured to the shafthaving a winding constructed by lap winding wiring into a number ofslots being a multiple of the number of pairs of poles, the slots beingformed on the outer circumferential surface of a core so as to extend inthe axial direction thereof; a commutator comprising a plurality ofsegments secured to an end portion of the shaft; and a plurality ofbrushes contacting the surface of the commutator, whereby the totalmagnetic attraction acting on the armature is zero and rotationalvibrations which cause operating noise do not arise, thereby enablingoperating noise to be reduced.

[0101] According to one form of the present invention, there is provideda motor for an electric power steering assembly wherein the number ofslots is a multiple of the number of pairs of poles and is not amultiple of the number of poles, enabling operating noise to be reduced,as well as reducing torque ripples and improving the feel of thesteering wheel to the driver.

[0102] According to one another form of the present invention, thecircuits of the armature are electrically connected to each other usingequalizing members, enabling the prevention of circulating currents fromflowing through the brushes due to differences in induced electromotiveforces arising between the circuits of the armature, thereby enablingthe commutating action of the brushes to be improved, and also enablingthe suppression of commutator sparks generated by the brushes.Furthermore, the magnitudes of both operating noise and torque ripplescan be reduced thereby.

[0103] According to still another form of the present invention, thereis provided a motor for an electric power steering assembly wherein thenumber of equalizing members is determined by Ns/(n×2)≦K≦Ns, where K isthe number of equalizing members, Ns is the number of slots in the core,and n is the maximum number of segments covered by the brushes, enablingthe appropriate number to be determined, thereby enabling theelimination of excess.

[0104] According to one form of the present invention, there is provideda motor for an electric power steering assembly wherein the currentpassing through the winding is controlled by pulse width modulation(PWM) driving, whereby the desired voltage can be applied with reducedoutput loss, and the size of the control unit can be reduced.

[0105] According to another form of the present invention, there isprovided a motor for an electric power steering assembly wherein thewiring is enamel-coated round wire, facilitating the mechanization ofthe step of winding the wiring onto the core, thereby enabling massproduction of the armature and reducing production costs.

[0106] According to still another form of the present invention, thereis provided a motor for an electric power steering assembly wherein themagnetic field portion comprises a plurality of permanent magnetsdisposed so as to be spaced around the inner wall of the yoke, enablingthe magnitude of torque ripples to be reduced. Reductions in size,improvements in the assembly operation, and cost reductions are alsoenabled.

What is claimed is:
 1. A motor for an electric power steering assemblycomprising: a yoke; a multi-polar magnetic field portion composed of atleast four poles secured to the inner wall of said yoke; a shaftdisposed within said yoke so as to be able to rotate freely; an armaturesecured to said shaft having a winding constructed by lap winding wiringinto an even number of slots formed on the outer circumferential surfaceof a core so as to extend in the axial direction thereof; a commutatorcomprising a plurality of segments secured to an end portion of saidshaft; and a plurality of brushes contacting the surface of saidcommutator.
 2. The motor for an electric power steering assemblyaccording to claim 1 wherein the number of said slots is even and is nota multiple of the number of said poles.
 3. A motor for an electric powersteering assembly comprising: a yoke; a multi-polar magnetic fieldportion composed of at least four poles secured to the inner wall ofsaid yoke; a shaft disposed within said yoke so as to be able to rotatefreely; an armature secured to said shaft having a winding constructedby lap winding wiring into a number of slots being a multiple of thenumber of pairs of said poles, said slots being formed on the outercircumferential surface of a core so as to extend in the axial directionthereof; a commutator comprising a plurality of segments secured to anend portion of said shaft; and a plurality of brushes contacting thesurface of said commutator.
 4. The motor for an electric power steeringassembly according to claim 3 wherein the number of said slots is amultiple of the number of pairs of said poles and is not a multiple ofthe number of said poles.
 5. The motor for an electric power steeringassembly according to claim 1 comprising equalizing members forpreventing circulating currents from flowing through said brushes due todifferences in induced electromotive forces arising between circuitswithin the circuits of said armature.
 6. The motor for an electric powersteering assembly according to claim 5 wherein Ns/(n×2)≦K≦Ns, where K isthe number of said equalizing members, Ns is the number of said slots insaid core, and n is the maximum number of segments covered by saidbrushes.
 7. The motor for an electric power steering assembly accordingto claim 1 wherein the current passing through said winding iscontrolled by pulse width modulation (PWM) driving.
 8. The motor for anelectric power steering assembly according to claim 1 wherein saidwiring is enamel-coated round wire.
 9. The motor for an electric powersteering assembly according to claim 1 wherein said magnetic fieldportion comprises a plurality of permanent magnets disposed so as to bespaced around the inner wall of said yoke.